The present invention relates to a phase-locked loop circuit (PLL) for digital signals and a control method for a data reproduction apparatus, and more particularly, to a phase-locked loop circuit and a data reproduction apparatus having such a structure that can adjust a slice level of a comparator and a phase of a PLL independently.
Generally, a circuit for reproducing digital data from an optical disc binarizes a signal from the disc by means of a comparator, and then generates a bit synchronous clock by means of a phase-locked loop circuit (hereinafter, referred to as PLL). In order to reduce a phase shift between the data signal and the bit synchronous clock, which is here called jitter, this reproduction circuit has a circuit for adjusting a slice level of the comparator and a phase of the PLL in an earlier time by a jitter detection means or the like. However, these adjustments for the slice level of the comparator and the phase of the PLL cannot be performed independently, without being affected with each other.
FIG. 5 is a diagram showing an example of a data reproduction apparatus having such construction. The conventional data reproduction apparatus shown in FIG. 5 includes a comparator 31, a PLL 35 that receives an output (data) from the comparator 31, and a phase shift detection means 34. The comparator 31 is provided with a slice level adjustment circuit 32. The PLL 35 includes a phase comparator 1, a phase adjustment circuit 5, a charge pump 2, and a voltage controlled oscillator (hereinafter, referred to also as a VCO) 3.
Hereinafter, an operation of the data reproduction apparatus will be described.
The comparator 31 receives a signal RF from a recording medium such as an optical disc, and binarizes the signal RF to output a binarized signal to the PLL 35 in the latter stage. Here, the slice level that is to be a threshold for the comparator 31 is adjusted by the slice level adjustment circuit 32 so as to minimize the amount of jitter (the phase shift between the data signal and the bit synchronous clock), in view of the amount of phase difference between the comparator output (data) and a phase-locked clock CLK.
The PLL 35 generates a clock CLK from the binarized signal (data) that is outputted from the comparator 31, and outputs the generated clock CLK. Here, the phases of the phase-locked clock CLK that is outputted from the VCO 3 and the data from the comparator 31 are compared with each other, and a control voltage for the VCO 3 is decided by means of the charge pump 2, and fed back to lock the phase.
The phase adjustment circuit 5 controls the amount of current in the charge pump 2 to prevent a phase difference from occurring when an inclination of a rising edge and an inclination of a falling edge of a PLL-generated clock become different from each other due to the difference in the amount of current between a PMOS transistor and a NMOS transistor constituting the charge pump 2. The amount of current is adjusted, for example, by measuring the amount of jitter to minimize the same.
FIG. 6 is a timing chart of the conventional data reproduction apparatus having the above-mentioned structure.
In order to increase the frequency of the phase comparison and improve the accuracy in the phase locking, the phase comparator 1 that constitutes the PLL 35 detects both of rising and falling edges of the data, and compares the respective edges with one edge (rising or falling edge) of the clock CLK. In the aforementioned structure in which the PLL 35 always performs the phase adjustment with the output from the comparator 31 whose slice level has been adjusted, the timing of the falling of the data changes according to duty variations resulting from the slice level adjustment for the data that is outputted from the comparator 31 as shown in FIG. 6, and accordingly the PLL is unfavorably locked so that the phase of a rising edge is also shifted. Thus, it is required to adjust the phase of the data every time when the slice level is adjusted. That is, the phase adjustment for the data cannot be performed before the adjustment of the slice level.
Further, in a state where neither the duty nor the phase is adjusted, it cannot be decided whether the phase shift between the data and the CLK is caused by the phase of the data or the duty. Therefore, it is impossible to perform only the duty correction according to the slice level adjustment, before performing the phase adjustment.
To be more specific, in cases of employing the phase comparator that performs the detection of both edges, it is impossible to perform one of the slice level adjustment and the phase adjustment independently in advance. Therefore, according to the prior art, the optimum slice level and phase must be decided on the basis of all combinations as to the two adjustment patterns.
When there are X possible increments in the slice level adjustment and Y possible increments in the phase adjustment, there are X*Y possible adjustment patterns in the entire data reproduction apparatus. For example when X and Y are 8 bits, i.e., there are 256 increments, respectively, there are 65,536 increments on the whole, and thus the processing time required for the adjustment becomes quite long. The initial adjustment including the jitter correcting control in this data reproduction apparatus is performed at each start of a disc, while it comes to be more difficult to increase the efficiency in the whole apparatus including other functional blocks, as the rotation speed of the optical disc is particularly increased.
Since the conventional phase-locked loop circuit and the control method for the data reproduction apparatus are constructed as described above, and the locking operation in the PLL varies depending on different slice levels of the comparator, the adjustment of the slice level and the phase of the PLL cannot be performed independently without being affected with each other, whereby the time required for the initial adjustment in the data reproduction apparatus cannot be reduced.
The present invention has for its object to provide a phase-locked loop circuit and a data reproduction apparatus, which can reduce a processing time that is required for initial adjustment in the data reproduction apparatus.
Other objects and advantages of the present invention will become apparent from the detailed description and specific embodiments described are provided only for illustration since various additions and modifications within the spirit and scope of the invention will be apparent to those of skill in the art from the detailed description.
According to a 1st aspect of the present invention, there is provided a phase-locked loop circuit comprising: a phase comparator for comparing phases of both rising and falling edges of binarized data with a phase of a bit synchronous clock; a charge pump for generating a control voltage from an output of the phase comparator; a voltage controlled oscillator for generating the bit synchronous clock from an output of the charge pump; and an edge switching means for controlling the phase comparator to perform switching between a comparison result as to only the rising or falling edge of the binarized data and a comparison result as to both of the edges, in the phase comparison between the binarized data and the bit synchronous clock by the phase comparator, and to output a selected result. Therefore, when this phase-locked loop circuit is incorporated into a data reproduction apparatus, an edge to be compared by the phase comparator can be changed, thereby adjusting the duty of the data signal without affecting a phase shift between the data signal and the bit synchronous clock, and further only the phase shift can be adjusted without depending on the duty.
According to a 2nd aspect of the present invention, in the phase-locked loop circuit of the 1st aspect, the phase comparator comprises: a first phase comparator for detecting the falling edge of the binarized data; and a second phase comparator for detecting the rising edge of the binarized data, and the edge switching means performs the phase comparison as to the both edges of the binarized data by using an OR of the results from the two phase comparators, and performs the phase comparison as to one of the edges by selecting one of the outputs from the two phase comparators and outputting the selected output. Therefore, when this phase-locked loop circuit is incorporated into a data reproduction apparatus, an edge to be compared by the phase comparator can be changed, thereby adjusting the duty of the data signal without affecting a phase shift between the data signal and the bit synchronous clock. Further, the adjustment of only the phase shift without depending on the duty can be realized by a simple construction and without requiring large circuit modification.
According to a 3rd aspect of the present invention, in the phase-locked loop circuit of the 1st aspect, the phase comparator comprises: a first phase comparator for detecting the falling edge of the binarized data; and a second phase comparator for detecting the rising edge of the binarized data, and the edge switching means performs the phase comparison as to the both edges of the binarized data by using an OR of outputs from the two phase comparators, and performs the phase comparison as to only one edge by fixing an output of one of the two phase comparators by means of a reset signal. Therefore, when this phase-locked loop circuit is incorporated into a data reproduction apparatus, an edge to be compared by the phase comparator can be changed, thereby adjusting the duty of the data signal without affecting a phase shift between the data signal and the bit synchronous clock. Further, the adjustment of only the phase shift without depending on the duty can be realized by a simpler construction and without requiring large circuit modification.
According to a 4th aspect of the present invention, there is provided a data reproduction apparatus including: a comparator for binarizing a reproduction signal from a disc to generate a data signal, a slice level adjustment circuit for adjusting a slice level of the comparator to correct a duty of the data signal, a phase-locked loop circuit comprising: a phase comparator for comparing phases of both rising and falling edges of the binarized data with a phase of a bit synchronous clock; a charge pump for generating a control voltage from an output of the phase comparator; a voltage controlled oscillator for generating the bit synchronous clock from an output of the charge pump; and an edge switching means for controlling the phase comparator to perform switching between a comparison result as to only the rising or falling edge of the binarized data and a comparison result as to both of the edges, in the phase comparison between the binarized data and the bit synchronous clock by the phase comparator, and to output a selected result, a phase adjustment circuit for controlling the charge pump included in the phase-locked loop circuit to correct the phase, and a phase shift detection means for controlling the duty correction and the phase correction. Therefore, the duty of the data signal can be adjusted without affecting a phase shift between the data signal and the bit synchronous clock, and further only the phase shift can be adjusted without depending on the duty.
According to a 5th aspect of the present invention, in the data reproduction apparatus of the 4th aspect, when there are X kinds of phase adjustment values and Y kinds of slice level adjustment values, one of the X kinds of phase adjustment values which minimizes a phase shift is selected, and thereafter one of the Y kinds of slice level adjustment values which optimizes the duty is selected. Therefore, the phase comparison as to one edge is performed only at the phase adjustment of the data reproduction apparatus, thereby reducing a processing time that is required for the control.